System and method for control of gas emissions from oil storage vessel

1. A system for control of emission of volatile gases from a holding tank for crude oil during unloading, loading and transport/holding of the oil, wherein a blanket gas is used to regulate a pressure and amount of combustible gas and to prevent ingress of oxygen in the holding tank for crude oil, the system comprising:

a recovery device configured to wash/clean, compress and condense hydrocarbon gas from the holding tank for crude oil, thereby producing recovered liquid hydrocarbons,

a storage tank configured to receive the recovered liquid hydrocarbons from the recovery device and to store the recovered liquid hydrocarbons under pressure, wherein the storage tank is arranged to also function as a separator, wherein liquid and gas are separated and any excess gas is vented out of the tank, and

an evaporation device configured to receive the recovered liquid hydrocarbons from the storage tank and to convert the recovered liquid hydrocarbons to gas form, whereby the hydrocarbon gas is used as the blanket gas in the holding tank for crude oil, wherein the excess gas from the storage tanks is led to a membrane separator where heaviest components in the excess gas are concentrated in a product flow which is re-circulated to an inlet side of the compressor, and wherein remaining excess gas is vented out through a regulating valve, and wherein the regulating valve is arranged to function as a pressure control valve which determines a storage pressure, condensation pressure, membrane pressure and outlet pressure of the compressor.

2. The system according to claim 1 , wherein the recovery device is connected to a manifold in the holding tank and comprises a compressor and a condenser arranged to cool the hydrocarbon gas with a cooling medium, whereby at least a part of the hydrocarbon gas is condensed to liquid at an operating pressure of the recovery device.

3. The system according to claim 2 , wherein the recovery device comprises a gas wash and a separator tank configured to clean hydrocarbon gas for particles and any other contaminations before the gas is fed into the compressor of the recovery installation and condenser, and wherein the crude oil which is being loaded is used as a gas wash medium.

4. The system according to claim 1 , wherein the evaporation device comprises a circulating heating medium which is heated with a number of heat exchangers, a throttle valve arranged to provide a pressure reduction in a mixture of liquid gas fed from the storage tank and the heating medium, whereby the liquid gas is evaporated, and a separator tank for separation and storage of the hydrocarbon gas.

5. The system according to claim 4 , wherein the heat exchangers are coupled in parallel or coupled in series, and comprise conventional heat exchangers.

6. The system according to claim 4 , wherein said heating medium in the evaporation device is arranged to transport an amount of internal energy sufficient to maintain the mixture of heating medium and evaporated gas above a critical temperature for freezing or hydrate formation after reduction in pressure, and wherein the heating medium comprises fresh water, seawater, heavier hydrocarbons, glycol, methanol, or a mixture of several materials.

7. The system according to claim 3 , further comprising:

a heat exchanger and a heating unit arranged in connection with the gas wash and the separator tank, wherein the heat exchanger is arranged to heat the crude oil that is loaded with the help of oil from the separator, wherein a valve is set up to control the direction of flow of the heating medium, and wherein the gas separator is set up to generate additional gas from the holding tank.

8. The system according to claim 3 , further comprising:

a gas separator arranged between the separator and the compressor, the gas separator comprising a gas cooler/condenser, a dew-point separator and a valve for control of the direction of flow, wherein the gas separator is arranged to control a dew-point of the generated gas.

9. A method to control emissions of volatile gases from a holding tank for crude oil during unloading, loading and transport/holding of the oil, the method comprising:

recovering volatile gas from crude oil in the holding tank,

storing the volatile gas as liquid hydrocarbon gas, wherein the storage tank functions as a separator, wherein liquid and gas are separated,

feeding the liquid hydrocarbon gas to an evaporation device, wherein the liquid hydrocarbon gas is stored in the storage tank before the liquid hydrocarbon gas is fed to the evaporation device,

supplying the liquid hydrocarbon gas with heat from a heating medium before a pressure reduction with a throttle valve,

converting the liquid hydrocarbon gas to gas form without the heating medium and evaporated gas coming down to a critical temperature for freezing or hydrate formation,

supplying the holding tank for crude oil with hydrocarbon gas as the blanket gas to regulate a pressure and amount of combustible gas and to prevent ingress of oxygen in the holding tank,

conducting excess gas from the storage tanks to a membrane separator where heaviest components in the excess gas are concentrated in a product stream,

recirculating the product stream to an inlet side of the compressor, and

venting remaining excess gas out through a regulating valve, wherein the regulating valve is arranged to function as a pressure control valve which decides the storage pressure, condensation pressure, membrane pressure and the outlet pressure of the compressor.

10. The method according to claim 9 , further comprising:

conducting hydrocarbon gas, heating medium and any remaining liquid gas to a separator, and

feeding the hydrocarbon gas after separation in the separator/tank to the holding tank for crude oil for application as the blanket gas.

11. The method according to claim 10 , further comprising:

feeding the heating medium to a heat exchanger after separation in the separator, with the help of a circulation pump,

increasing a temperature is increased with heat exchanging with a suitable heating source, and

recirculating the heating medium after heating and again mixed with the liquid gas.

12. The method according to claim 9 , further comprising:

before the liquid hydrocarbon gas is fed to the storage tank, recovering with a recovery device hydrocarbon gas from the holding tank for crude oil, wherein the recovery device comprises a compressor configured to pressurize the gas, and a heat exchanger configured to cool the gas with the help of a cooling medium, whereby at least a part of the gas is condensed to liquid at the actual pressure, and is fed to the storage tank.

13. The method according to claim 12 , wherein the recovery device comprises a gas wash to clean the hydrocarbon gas for particles and any other contaminants before the gas is fed to the recovery installation and a separator to separate oil and gas, and wherein the crude oil that is being loaded is used as the gas wash medium.

14. The method according to claim 9 , wherein the recovery device comprises a heat exchanger and a heating unit, and wherein the heat exchanger heats the crude oil which is being loaded with oil from the separator.

15. The method according to claim 9 , wherein the method is carried out during unloading, loading and transport/holding of oil in an oil tank on board an oil tanker.

16. The method according to claim 15 , wherein the recovery device comprises a gas separator encompassing a gas cooler/condenser, a dew-point separator and a valve for control of the direction of flow, and wherein the gas separator generates additional gas from the holding tank.

17. The method according to claim 9 , wherein the hydrocarbon gas, heating medium and any remaining liquid gas are conducted to a gravity separator, or one or more cyclone separators or centrifuges.

18. The method according to claim 11 , wherein the suitable heating source comprises seawater.

19. The system according to claim 1 , wherein the unloading, loading and transport/holding of the oil is from/to an oil tank on board an oil tanker.

20. The system according to claim 4 , wherein the heat exchangers comprise plate heat exchangers, tube bundle heat exchangers, air coolers, evaporative air coolers, or other types of conventional heat exchangers.

21. A system for control of emission of volatile gases from a holding tank for crude oil during unloading, loading and transport/holding of the oil, wherein a blanket gas is used to regulate a pressure and amount of combustible gas and to prevent ingress of oxygen in the holding tank for crude oil, the system comprising:

a recovery device configured to wash/clean, compress and condense hydrocarbon gas from the holding tank for crude oil, thereby producing recovered liquid hydrocarbons,

a storage tank configured to receive the recovered liquid hydrocarbons from the recovery device and to store the recovered liquid hydrocarbons under pressure, and

an evaporation device configured to receive the recovered liquid hydrocarbons from the storage tank and to convert the recovered liquid hydrocarbons to gas form, whereby the hydrocarbon gas is used as the blanket gas in the holding tank for crude oil, wherein the evaporation device comprises a circulating heating medium which is heated with a number of heat exchangers, a throttle valve arranged to provide a pressure reduction in a mixture of liquid gas fed from the storage tank and the heating medium, whereby the liquid gas is evaporated, and a separator tank for separation and storage of the hydrocarbon gas.

22. The system according to claim 21 , wherein the heat exchangers are coupled in parallel or coupled in series, and comprise conventional heat exchangers.

23. The system according to claim 21 , wherein said heating medium in the evaporation device is arranged to transport an amount of internal energy sufficient to maintain the mixture of heating medium and evaporated gas above a critical temperature for freezing or hydrate formation after reduction in pressure, and wherein the heating medium comprises fresh water, seawater, heavier hydrocarbons, glycol, methanol, or a mixture of several materials.

24. The system according to claim 21 , wherein the heat exchangers comprise plate heat exchangers, tube bundle heat exchangers, air coolers, evaporative air coolers, or other types of conventional heat exchangers.

25. A method to control emissions of volatile gases from a holding tank for crude oil during unloading, loading and transport/holding of the oil, the method comprising:

recovering volatile gas from crude oil in the holding tank,

storing the volatile gas as liquid hydrocarbon gas,

feeding the liquid hydrocarbon gas to an evaporation device,

supplying the liquid hydrocarbon gas with heat from a heating medium before a pressure reduction with a throttle valve,

converting the liquid hydrocarbon gas to gas form without the heating medium and evaporated gas coming down to a critical temperature for freezing or hydrate formation,

supplying the holding tank for crude oil with hydrocarbon gas as the blanket gas to regulate a pressure and amount of combustible gas and to prevent ingress of oxygen in the holding tank,

conducting hydrocarbon gas, heating medium and any remaining liquid gas to a separator, and

feeding the hydrocarbon gas after separation in the separator/tank to the holding tank for crude oil for application as the blanket gas.

26. The method according to claim 25 , further comprising:

feeding the heating medium to a heat exchanger after separation in the separator, with the help of a circulation pump,

increasing a temperature is increased with heat exchanging with a suitable heating source, and

recirculating the heating medium after heating and again mixed with the liquid gas.